I would keep the G23 and purchase .357 Sig and 9mm Bbls. Keep in mind you need to purchase G19 magazines to run 9mm reliably in the G23, but the .40 S&W mags are compatible with .357 Sig ammo.
Lone Wolf does make black and stainless barrels in 40 to 9 conversion, but they are threaded for a suppressor. Their standard length conversion barrels are stainless.
I own a G23 and am ordering a threaded 9mm barrel from Lone Wolf for it.

